The present invention relates to high frequency switchmode half-bridge inverters for asymmetrical loads and specifically to high frequency electronic ballasts for gas discharge devices. More specifically, the present invention relates to high frequency electronic ballast for high intensity discharge (HID) lamps.
The prior art is replete with many known half-bridge inverters providing high frequency ballast for gas discharge lamps, especially for HID lamps. For instance, high efficient electronic ballasts based on half-bridge inverter configuration which can be used with HID (HPS) lamps are U.S. Pat. No. 5,313,143 entitled “Master-slave half-bridge DC-to-AC switchmode power inverter” (See also a paper entitled “Master-Slave Half-Bridge Inverter” presented at APEC'93); U.S. Pat. No. 5,229,927, entitled “Self-symmetrizing and self-oscillating half-bridge power inverter”, and U.S. Pat. No. 6,329,761, entitled “Frequency controlled half-bridge inverter for variable loads” from the same inventor of the present invention. Further applications of half-bridge inverters are U.S. Pat. No. 5,253,157, entitled “Half-bridge inverter with capacitive voltage equalizer” from Severinsky; U.S. Pat. No. 6,242,867, entitled “Circuit for synchronizing the ignition of electronic ballast discharge lamps” from Pogadaev, and U.S. Pat. No. 5,932,976, entitled “Discharge lamp driving” from Maheshwari, especially for ignition methods. Some of the cited inventions provide solution for equalizing the voltages of the voltage divider capacitors of a half-bridge inverter, but none of these inventions solves a specific problem related to the startup process of a HID lamp, especially for metal halide lamps, described in the following part.
An important application of the switchmode power inverters is supplying gas discharge devices, especially high intensity discharge (HID) lamps in the range of 35W to 400W. In this case, the load impedance of the inverter is a HID lamp connected in series with an inductor. At high frequency powering of a HID lamp, the interaction between the ballast and the lamp is more sensitive than that of a conventional low frequency (50/60 Hz) ballast. During the startup process, including the transition from glow to arc discharge, HID lamps may have asymmetrical impedance resulting unequal voltages of the voltage divider capacitors of the half-bridge inverter (See FIG. 8).
For instance, let V2>V3, where V2+V3=V1=constant. At the end of the startup process, the lamp goes into arc discharge state having symmetrical impedance. Therefore, if the ON-times of the main switches are equal, a transient process starts, and after a certain time interval, the equilibrium V2=V3 is achieved. During this transient process high current peak occurs (V2>V3) which can damage the main switches of the inverter. Furthermore, the inductor may be also saturated causing an extra current peak exceeding the maximum allowable current peak values of the main switches.
The present invention provides a protection, namely a dynamic solution for the limitation of high current peaks during the startup process.
Furthermore, the present invention introduces a different, and more effective ignition solution than the ignition solution of U.S. Pat. No. 6,329,761, providing essentially less stress for the main switches of a half-bridge inverter.
Also, the present invention provides a special MOSFET driver solution for the main switches of a half-bridge inverter, wherein the main switches can be simultaneously switched off applying an more effective solution comparing to the MOSFET drivers applied in the mentioned patent applications, for instance in U.S. Pat. No. 6,329,761, wherein the main switches can be simultaneously switched off slowly, causing high dissipation in the switches at inductive load, therefore, they are incapable for fast and repeating current protection caused by an asymmetrical load during the startup process.
Furthermore, the theoretical background for the practical lamp power control, where the lamp is connected in series with an inductor and supplied by a square wave inverter (push-pull, half-bridge, or full bridge) resulting a special ballast curve can be found in a paper of J. Melis, entitled “Ballast Curves for HPS Lamps Operating on High Frequency” (IAS' 92).